The invention relates to a valve for controlling liquids as defined in further detail in the preamble to claim 1.
From European Patent.Disclosure EP 0 477 400 A1, a valve is already known which is actuatable via a piezoelectric actuator. This known valve has an arrangement for an adaptive mechanical tolerance compensation, effective in the stroke direction, for a travel transformer of the piezoelectric actuator, in which the deflection of the piezoelectric actuator is transmitted via a hydraulic chamber.
The hydraulic chamber, which functions as a so-called hydraulic step-up means, encloses a common compensation volume between two pistons defining this chamber, of which one piston is embodied with a smaller diameter and is connected to a valve member to be triggered, and the other piston is embodied with a larger diameter and is connected to the piezoelectric actuator. The hydraulic chamber is fastened between the two pistons in such a way that the actuating piston of the valve member, which piston is retained in its position of repose by means of one or more springs relative to a predetermined position, executes a stroke that is increased by the step-up ratio of the piston diameter when the larger piston is moved a certain travel distance by the piezoelectric actuator. The valve member, piston and piezoelectric actuator are located one after the other on a common axis.
Via the compensation volume of the hydraulic chamber, tolerances caused by temperature gradients in the component or different coefficients of thermal expansion of the materials used and possible settling effects can be compensated for without causing a change in position of the valve member to be triggered.
Compensating for changes in length of the piezoelectric actuator, the valve member or the valve housing by means of the hydraulic chamber disposed between two pistons requires a complicated construction, however, and is problematic in terms of the incident leakage losses and the refilling of the hydraulic chamber. Specifically, the hydraulic coupler requires a system pressure, which drops because of leakage if adequate refilling with hydraulic liquid is not done.
In the industry, in common rail injectors, versions are known in which the system pressure is expediently generated in the valve itself, and a constant system pressure is assured even upon system starting. To that end, hydraulic liquid is drawn from a high-pressure region of the fuel to be controlled and delivered to the low-pressure region at the system pressure. This is done with the aid of leakage gaps, which are defined by leakage or filling pins. However, the leakage losses are often undesirably high, which is a disadvantage.
In versions with a hydraulic step-up means, the highest possible system pressure in the low-pressure region is advantageous. However, then it is often a problem that sealing elements, which seal off the piezoelectric actuator, which as a rule is not fuel-resistant, from the low-pressure region, can withstand only slight pressure, and therefore rapid refilling at elevated pressure cannot be employed.
The object of the invention is to create a valve for controlling liquids which in particular allows slight leakage from the low-pressure region and enables filling of a hydraulic step-up means at elevated pressure.
The valve of the invention for controlling liquids and having the characteristics of claim 1 has the advantage that upon triggering of the piezoelectric unit, only an extremely slight volume is positively displaced out of the system pressure region, so that the continuous leakage of the system, with the valve of the invention, is reduced to a minimum, yet a continuous flow through the hydraulic chamber and thus flushing out of any air that may have entered it are always assured.
The refilling of the hydraulic chamber can advantageously take place at high pressure, so that the fastest possible refilling is achieved. Especially with a valve embodied as a fuel injection valve, the time interval between fuel injections can thus be kept very slight, and as a result high engine speeds can be achieved.
Furthermore, the volume having the system pressure in the low-pressure region can be made quite small. This shortens the required time for filling the system region upon system starting.
A significant advantage of the invention is also that because of the separate disposition of the system pressure chamber, pressure surges on a sealing element that is optionally provided between the low-pressure region and the piezoelectric unit are avoided. Thus in the valve of the invention, the service life of the sealing element is favorably affected, and the magnitude of the system pressure is not limited by the sealing element.
Since a pumping effect with the sealing element and resultant high leakage losses from the system pressure region upon triggering of the piezoelectric unit are averted, the piston diameter and the sealing element can be selected freely without having to take into consideration a possible effective pumping area of the sealing element.
Further advantages and advantageous features of the subject of the invention can be learned from the specification, drawing and claims.